Coating containing urethane and acrylic resin mixture for improved resistance to chipping

Abstract: The present invention is a coating composition including a resin mixture of polyurethane resin and acrylic resin and a crosslinking agent. The resulting resin has improved flexibility and resistance to stone chipping.

Description: Field of the Invention The present invention is directed to coating compositions. More specifically, the present invention is directed to coating compositions containing hydroxy functional polyurethane resins and acrylic resin. Background of the Invention It is well known in the automotive industry to apply primer surfacer coatings and anti-chip coatings to areas of a vehicle which are prone to stone chipping. Coatings for this purpose have conventionally been unsaturated polyester based coatings. It is desirable to utilize acrylic based primer surfacer and anti-chip coatings because they provide a primer layer that is resistant to degradation by ultraviolet radiation. UV radiation causes intercoat degradation of the primer layer which causes a loss of adhesion between the primer coating and other coating layers and results in delamination. However, the use of such coatings is problematic, as acrylic based coatings are more brittle and less chip resistant than polyester systems. It has been discovered that the addition of a urethane diol resin to an acrylic resin provides a coating with increased flexibility and greatly improved resistance to stone chipping. Summary of the Invention The present invention is a coating composition including a hydroxy functional polyurethane resin and an acrylic resin. The hydroxy functional polyurethane resin is the reaction product of a polyol and an unblocked aliphatic isocyanate. The polyurethane resin is used in acrylic based coating compositions to improve flexibility and resistance of the coating to stone chipping. Detailed Description of the Invention The present invention is a coating composition including an acrylic resin and a polyurethane resin which is the reaction product of a hydroxy functional polyol and an unblocked aliphatic diisocyanate, for improved flexibility of the coating and for improved resistance of the coating to stone chipping. The polyurethane resin of the present invention is useful in powder coatings, as well as in solvent-borne and water-borne coatings. For purposes of the present invention, the polyurethane is the reaction product of hydroxy functional polyols including alkyl polyols, polyester polyols, polyether polyols and acrylic polyols. Polyester polyols can be prepared by ring-opening polymerization of a cyclic ester, such as caprolactone or butyrolactone. Particularly preferred are caprolactone based polyols having a molecular weight of between 400 and 3,000 and a hydroxyl number of 250 to 30 mg KOH/g. The polyol is reacted with an unblocked aliphatic diisocyanate having the formula NCO-R-NCO, where R is an aliphatic linkage having a carbon chain length ranging from C1 to C20. Particularly useful diisocyanates for the present invention include methylene dicyclohexyl diisocyante, hexamethylene diisocyanate 2-methyl-1,5-pentane diisocyanate, 2,2,4-trimethyl-1,6-hexane diisocyanate, 1, 12-dodecane diisocyanate and isophorone diisocyanate (IPDI). Aliphatic isocyanates are preferred for purposes of the present invention, as the aromatic isocyanates are less resistant to degradation by ultraviolet radiation. When the urethane resin is to be used in a powder coating composition, the preferred diisocyanate includes cyclohexane derivatives, as the cyclohexane ring is capable of forming conformational isomers. The ability of the cyclohexane ring to change conformation is thought to provide additional flexibility necessary for successful incorporation of the urethane resin in a powder coating. The preferred isocyanate for powder coatings is methylene dicyclohexyl diisocyante, sold under the trademark Desmodur® W from Miles Inc., Mobay Road, Pittsburgh, Pennsylvania 15205. The polyurethanes are produced by reacting the diol component and the isocyanate component so as to result in the desired molecular weight properties. The components should be reacted in an amount such that the ratio of the number of isocyanate equivalents of the isocyanate component to the number of equivalents of hydroxyl groups in the diol component is in the range of from 1.0 : 2.0 to 9 :10, preferably from 3:4 to 5:6. Preferably, the resultant polyurethane resin has a molecular weight ranging from 300 to 5,000. The components are reacted at a temperature of between 15°C to 125°C, optionally in the presence of a polyurethane catalyst. The polyurethane catalyst used depends upon the particular components and reaction conditions. For purposes of the present invention the preferred catalyst is di-butyl tin di-laurate. Acrylic polymers suitable for the present invention are well known in the art, and can be prepared from monomers such as methyl acrylate, acrylic acid, methacrylic acid, methyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, and the like. The functional group that is reactive with curing agent e.g. hydroxyl, can be incorporated into the ester portion of the acrylic monomer. For example, hydroxy-functional acrylic monomers that can be used to form such polymers include hydroxyethyl acrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, hydroxypropyl acrylate and the like. Amino functional acrylic monomers would include t-butylaminoethyl methacrylate and t-butylaminoethylacrylate. Other acrylic monomers having reactive functional groups such as epoxy functional groups would include glycidyl methacrylate. The polyurethane resin is utilized in the resin mixture in an amount between 5 and 45 % by weight, and the acrylic resin is utilized in an amount between 55 and 95 % by weight based on total weight of the resin mixture. Preferably, the polyurethane resin is utilized in an amount between 20 and 40 % by weight, and the acrylic resin is utilized in an amount between 60 and 80 % by weight based on total weight of the resin mixture. When the polyurethane flex resin is used in an acrylic powder coating composition, the preferred acrylic resins are hydroxy functional and have a hydroxyl value of 20 to 120 and a number average molecular weight of 3,000 to 35,000. The polyurethane resin is added to an acrylic resin by melting the polyurethane resin and then adding the acrylic resin to it in small portions, allowing the acrylic resin to melt into the polyurethane resin. The resin mixture can then be used in a powder coating composition. Powder coating compositions may be formulated from the polyurethane and acrylic resin mixture and suitable crosslinking agents. Suitable crosslinking agents include any one of a number of well known hydroxy group reactive crosslinking agents, such as polyisocyanates, and/or aminoplast resins. Particularly preferred curing agents are blocked aliphatic isocyanate crosslinkers such as e-caprolactam blocked isophorone diisocyanate. Solvent-borne coating compositions are formulated by adding the polyurethane flex resin to an acrylic solvent-borne coating composition. Suitable solvents are included to disperse the polyurethane resin in the acrylic resin. Suitable crosslinkers are also included in the coating composition. Examples of useful crosslinkers include blocked melamine crosslinking agents. Particularly useful are monomeric and polymeric melamine crosslinkers with high or low imino content. The polyurethane flex resin is also useful with water-borne acrylic based coating compositions containing an acrylic copolymer. The urethane resin is reacted with a compound having a functional group to provide anionic, cationic or non-ionic stabilization. One example of this, is the anionic stabilization provided by reaction of the urethane with an acid functional diol to form a hydroxy terminated, acid stabilized polyester. A useful acid for this purpose is dimethyolpropionic acid. The resin can then be at least partially neutralized with an amine and added to the acrylic based water-borne coating composition. Any suitable crosslinker may be utilized. Such crosslinkers include aminoplast and polyisocyanate crosslinkers. The invention is further illustrated by the following non-limiting examples. Examples Example 1 Synthesis of Polyurethane Powder Resin A 5 liter steel reaction vessel was charged with 870 g 3.26 eq) caprolactone based polyol and 1.0 g dibutyl tin dilaurate. This mixture was slowly heated to 50°C in order to melt out the polyol and then 305 g (2.175 eq) dihexamethylene diisocyanate was added slowly, over a two hour period, such that the exothermic reaction was held below 100°C. At this stage, the reaction mixture was held at 100°C for 30 minutes and a further charge of 40 g (0.28 eq) dihexamethylene diisocyanate was added. At this point the reaction temperature rose to 120°C and was held there for 30 minutes. At this stage a further charge of 20g (0.14 eq) dihexamethylene diisocyanate was added and the temperature maintained at 120°C for an hour. At this time an NCO titration revealed no free isocyanate in the reaction mixture, and the product was then isolated as a waxy semi-solid. Example 2 Preparation of the Polyurethane-Acrylic Resin Blend for Powder Coating Composition 750 g of the polyurethane resin from Example 1 was charged to the reactor and heated to 130°C to melt it. When the polyurethane was melted, 2150 g of an acrylic resin, sold under the trademark Joncryl® 587, (from S. C. Johnson and Son, Racine, Wisconsin 53403), was added to the reactor in small portions and allowed to melt into the polyurethane resin. The complete charge of acrylic resin was added over 4 hours. At the end of this time the resin was isolated for use in coating compositions. Example 3 Powder Coating Compositions Prepared With and Without the Resin Blend from Example 2 Ingredients Control No Resin Blend Composition with Resin Blend Resin Blend (from Ex. 2) --- 38.58 g Acrylic Resin 36.12 g --- Blocked IPDI 23.83 g 21.43 g Acid Acrylic Resin 1.0 g 1.0 g Flow Additive 1.00 g 1.00 g Benzoin 0.40 g 0.40 g Lanco Wax 0.40 g 0.40 g White Pigment 18.52 g 18.52 g Black Pigment 0.10 g 0.10 g Filler 18.52 g 18.52 g Fumed Silica 0.05 g 0.05 g Total 100.0 100.0 The above ingredients were added to a premixer in the order given and mixed for 60 seconds. The mixture was extruded in a Werner and Pfleiderer 2SK 30 extruder at 250 RPM and at 130°C. Coatings were applied with an ONODA spray gun, Model No. GX108. Panels were sprayed with the urethane acrylic resin mixture to a thickness of between 2.0 and 10 mils over electrocoated cold rolled steel (CRS). The panels were baked for 30 minutes at 350°F. Lower cure materials can be made by adjustment to the powder formulation. Where desired applications of basecoat and clearcoat are made and baked for 30 minutes at 265°C.

Claims: A powder coating composition comprising a) a resin mixture of i) a polyurethane resin having a functional group selected from hydroxy, amino and epoxy functionalities, wherein the polyurethane resin has a molecular weight ranging from 300 to 5,000 and ii) an acrylic resin having a functional group selected from hydroxy, amino and epoxy functionalities and having a measured T g of between 30° and 60°C and b) a crosslinking agent, reactable with the functional groups on the polyurethane and acrylic resins. The coating composition of claim 1, wherein the polyurethane is obtained by reaction of a hydroxy functional polyol selected from alkyl polyols, polyester polyols, polyether polyols and acrylic polyols and isocyanate selected from methylene dicyclohexyl diisocyante, hexamethylene diisocyanate, 2-methyl-1,5-pentane diisocyanate, 2,2,4-trimethyl-1,6-hexane diisocyante, 1,12-dodecane diisocyanate and isophorone diisocyanate. The coating composition of claim 1, wherein the acrylic resin is hydroxy functional and has a molecular weight ranging from 3,000 to 35,000 and has a hydroxyl value of 20 to 120. The coating composition of claim 1, wherein the polyurethane resin is present in an amount between 5 and 45% by weight, and the acrylic resin is present in an amount between 55 and 95 % by weight based on total weight of the resin mixture. The coating composition of claim 1, wherein the polyurethane resin is present in an amount between 25 and 45% by weight, and the acrylic resin is present in an amount between 55 and 75 % by weight based on total weight of the resin mixture. The coating composition of claim 1, wherein the acrylic resin is hydroxy functional and the urethane resin is hydroxy functional. The coating composition of claim 6, wherein the crosslinking agent is a polyisocyanate resin or aminoplast resin. The coating composition of claim 1, wherein the polyurethane resin is obtained by reaction of a hydroxy functional polyol and an unblocked aliphatic polyisocyanate. The coating composition of claim 8, wherein the polyurethane has a ratio of isocyanate equivalents of the isocyanate component to equivalents of hydroxyl groups in the polyol component ranging from 1.0 : 2.0 to 9 :10. The coating composition of claim 1, wherein the coating composition is used as a primer coating. The coating composition of claim 8, wherein the acrylic resin is a hydroxy functional acrylic resin. The coating composition of claim 8, wherein the urethane resin is the reaction product of dicyclohexyl diisocyanate and a caprolactone polyol. The coating composition of claim 8, wherein the crosslinking agent is selected from polyisocyanate and aminoplast resins. A powder coating composition comprising a) a mixture of i) a hydroxy functional polyurethane resin obtained by reaction of a hydroxy functional polyol and an unblocked aliphatic isocyanate, wherein the polyurethane resin has a molecular weight ranging from 300 to 4,000 and ii) a hydroxy functional acrylic having a Tg of between 30° and 60 °C and b) a crosslinking agent reactable with the hydroxy functional polyurethane and hydroxy functional acrylic resins. The powder coating composition of claim 14, wherein the polyurethane is obtained by reaction of a hydroxy functional polyol selected from alkyl polyols, polyester polyols, polyether polyols and acrylic polyols and isocyanate selected from methylene dicyclohexyl diisocyante, hexamethylene diisocyanate, 2,2,4-trimethyl-1,6-hexane diisocyante, 1,12-dodecane diisocyanate and isophorone diisocyanate. The powder coating composition of claim 14, wherein the crosslinking agent crosslinking agent is a polyisocyanate or aminoplast resin.

Inventors: FOUKES RICHARD J, HARRIS PAUL J, FOUKES, RICHARD J., HARRIS, PAUL J.
Applicants: BASF CORP, BASF CORPORATION
Application year: 1994
Main classification: C08G18
IPC codes: C09D175, C09D5, C08G18, C09D133